The invention relates to a tensioner with a damping mechanism and a belt drive system that includes such a tensioner. More particulary, the invention relates to a tensioner with a spring that biases the position of a pivot-arm to which a belt engaging pulley is rotatably mounted. The tensioner of the invention with its damping mechanism is particularly useful in controlling tension of a micro-V belt drive system such as a front end accessory drive for automotive engine applications.
A mechanical tensioner is used to automatically control tension of a micro-V belt of a front end accessory drive for automotive engine applications. Usually, such a tensioner has a pivot-arm that rotates about a pivot secured to a base and uses a sleeve type bushing on the pivot to provide a bearing surface for the rotating pivot-arm. Many of such bearings are made of plastic and are subject to wear over the expected life of the tensioner. A torsion spring is often used with one end connected to the base and another end connected to the pivot-arm to bias the position of the pivot-arm and position an attached pulley against a belt. The spring is also used to generate a spring force operative with a damping mechanism that generates a normal force component to a friction sliding surface to inhibit or dampen oscillatory movements of the pivot-arm.
A common tensioner design solving bulk, cost and damping problems is disclosed in U.S. Pat. No. 4,473,362. The ""362 tensioner has a pivot-arm attached to an off-set cylindrical member that supports the pivot-arm and rotates about a pivot secured to a base. Only one torsion spring is used with one end connected to the pivot-arm and the other end connected to the base. A single sleeve-type bushing on the pivot has a bearing surface that supports the cylindrical member. The radial plane of a pulley bearing is off-set in relation to the sleeve-type bushing which introduces a moment or couple as a load which must be carried by the bushing. Such tensioners are referred to as xe2x80x9cZedxe2x80x9d type tensioners because of the off-set of the pulley relative to its support.
The belt of a drive system utilizing such Zed type tensioners engages the pulley and generates a belt force at the pulley which is transmitted to the cylindrical member. As explained in the ""362 patent, the unequal loads to the bushing are reduced by a damping means that generates a normal force component acting in generally the same direction as the belt force component. In some cases, the normal force component of the damping means is not sufficient to balance against a moment generated by the belt force being off-set, as such the single bushing has a tendency to wear unevenly and prematurely.
A damping mechanism typically used in such xe2x80x9cZedxe2x80x9d type tensioners is disclosed in U.S. Pat. No. 5,632,697. The ""697 damping mechanism includes a damping means where the normal force generated by the damping means is greater than the spring force component activating the damping mechanism. The damping means has a brake shoe with an outer arcuate friction surface that engages the inside of a second cylindrical member to provide friction surface sliding therewith. The brake shoe has two oppositely facing inside ramp surfaces where one of the ramp surfaces slideably engages a complimentary ramp surface of the base and the other ramp surface slideably engages an extension of the spring end which applies the spring force to the brake shoe. The spring force is generated by an end of the spring bending around a proturbance formed in the base. The spring under torsion applies a spring force substantially normal to the inside ramp surface pressing ramp surface of the shoe against the complimentary ramp surface of the base and pressing a lining against a the compliementary inside surface of the second cylindrical member thereby causing a reactionary force to be imparted to the shoe.
Since the ""697 damping mechanism requires the use of multiple ramp surfaces, the brake shoe and a proturbance in the base the ability to achieve greater damping is limited. In addition, the use of multiple parts increases the cost, weight and size of the overall tensioner design.
In accordance with the invention, a damping mechanism is provided that is particularly useful in micro-V belt front end accessory drive systems utilizing a tensioner in automotive applications where the pulley alignment over the expected life of the tensioner is important and the overall cost, weight and size of the tensioner is important. The damping mechanism of the invention includes a spring having first and second ends with the first end operatively connected to a damping plate having first and second spring contact points whereby the spring torque applied to the damping plate in combination with a reactive force at the pivot surface generates a normal force. The damping plate also includes an outer friction surface and at least one ramp surface. In one embodiment of the invention the second end of the spring is connected to a tensioner base. In another embodiment of the invention the second end of the spring is connected to a tensioner pivot-arm.
An advantage of the invention is that a reduced bulk damping mechanism is provided having a reduced number of component parts and an increased area of friction contact. Another advantage of the invention is provided by reducing the need for expensive machined proturbances and ramp surfaces in either the base or the pivot-arm.